The particular drilling process used may be referred to as "pecking" wherein the drill bit cuts a hole part-way into the workpiece, withdraws out of cutting contact with the bottom of the hole and preferably completely clear of the hole (thus clearing cuttings and cooling the bit), and then advances to the bottom of the hole again to resume cutting.
The pecking cycle includes the steps of:
(1) rapidly advancing the drill bit to a position near the workpiece;
(2) upon reaching a position near the workpiece, changing the advance speed of the drill bit to an appropriate speed for drilling;
(3) drilling a hole in the workpiece to a portion of the desired depth for the hole;
(4) retracting the drill bit from the bottom of the hole after an amount of drilling (in a preferred embodiment, after a predetermined time) to a second predetermined position (in a preferred embodiment, just clear of the workpiece);
(5) advancing the drill bit toward the bottom of the hole;
(6) upon reaching a position near the bottom of the hole, changing the advance speed of the drill bit to an appropriate speed for drilling;
(7) repeating steps (3) through (6l) until the desired depth of the hole is achieved; and
(8) retracting the drill bit to clear the workpiece.
Pecking is the preferred method to drill a hole to help remove the cuttings which congregate in the flutes of the drill bit, and it is very useful in the drilling of composite materials such as those found in the aircraft industry. When drilling through the materials normally used in those composites, a cooling liquid applied to the drill bit can make an undesirable mud when it combines with the cuttings. A pecking drill can drill without liquid coolant, because the bit cools upon the frequent withdrawals from the workpiece. Also, when drilling from a soft layer into a hard layer of laminated materials, the hard cuttings mar the hole formed in the softer material if they are not removed. Pecking removes those hard cuttings faster than non-pecking drilling methods.
Originally, the pecking method was done manually with a drill press or portable hand drill. Later, automatic peck drills were made using air cylinders to advance and retract the bit, usually by advancing and retracting the entire drill, including the motor used to turn the spindle. This resulted in large and unwieldy drills.
In the pneumatic pecking drills, a hydraulic hydro-check restrains the air cylinder that is stroking the bit into the workpiece. An adjustable by-pass or restriction controls the rate of advancement toward the workpiece. Changing the by-pass controls the rate of advancement of the drill bit, and it opens to permit rapid retraction when removing the bit from the hole.
Pneumatic peck drills are unsatisfactory when the feed rate must be accurately controlled, because the air cylinder produces a surge when the bit passes through the opposite face of the material being drilled or from a hard region to a softer one in a laminated structure. This surge increases the amount that cuttings from the hard region intrude into the soft region, and it produces a burr on the back side of the workpiece when breaking through.